DESCRIPTION: (Verbatim from the Applicant's Abstract) The long-term objective of this research is to gain a better understanding of the basic mechanisms of epilepsy. The perforant path stimulation (PPS) model will be used to determine the anatomical and physiological significance of basal dendrites on dentate granule cells following status epilepticus. In this model, mossy fiber sprouting occurs and provides a basis for the formation of recurrent excitatory connections among granule cells. The discovery of basal dendrites on granule cells that enter the hilus where mossy fibers are concentrated may provide an additional postsynaptic target for mossy fibers, and it could increase the amount of recurrent excitation. We will test the hypothesis that hippocampal status epilepticus induces the formation of basal dendrites on dentate gyrus granule cells that become integrated in to hippocampal circuitry. The first specific aim will determine whether seizures are a sufficient stimulus to induce the formation of basal dendrites on granule cells. An array of staining methods will be used to demonstrate the time course of basal dendrite formation in the PPS model and determine whether seizures in three other models cause basal dendrite formation. In addition, this aim will determine if hilar cell death without status epilepticus is a sufficient stimulus to induce basal dendrite formation. Global ischemia will be used to cause hilar neuronal loss. The second aim will determine if the basal dendrites on granule cells from PPS rats are anatomically integrated into hippocampal circuitry and receive identified synaptic inputs. Electron microscopic preparations will be used to identify the basal dendrites of granule cells to determine whether they are postsynaptic to mossy fibers and GABAergic axon terminals. The third aim will determine if the basal dendrites on granule cells from PPS rats are functionally integrated into hippocampal circuitry. Specifically, do basal dendrites possess functional receptors for excitatory and inhibitory neurotransmitters and does activation of these receptors affect somatic excitability of the granule cells and their ability to generate action potentials? Patch-clamp recordings from the somata or basal dendrites will be made after identifying granule cells with these dendrites using DiI loading in slices from epileptic rats. The fourth aim will determine whether basal dendrites sprout from mature granule cells or if they are immature dendrites typically observed for newly generated granule cells. These studies will provide new information about a novel neuroplastic change observed in the hippocampal dentate gyrus o rats with temporal lobe epilepsy, and are relevant to epileptic humans who have double the number of granule cells with basal dendrites.